Biology Reference
In-Depth Information
a clear brood hierarchy, the smallest chicks quickly died, leaving parents with a reduced
brood which they could raise effectively. In the synchronous treatment, nestlings took
longer to die and more died. Under good conditions, however, synchronous broods tended
to do best (Table 8.5). Further work is needed to make Lack's hypothesis quantitative
rather than qualitative.
Sex ratio conflict
If sibling rivalry might sometimes benefit parents, how then are we to test for parent-
offspring conflict? The ideal test would be to calculate the optimal level of parental
investment for both parent and offspring, and to show that these differ (Fig. 8.8). This
has been demonstrated just once, in a brilliant paper by Robert Trivers and Hope Hare
(1976) which revealed parent-offspring conflicts over offspring sex ratio in social
hymenoptera.
This is explained in detail in Chapter 13; just a brief summary is provided here.
Sex in hymenoptera is determined by haplodiploidy. Fertilized eggs produce
daughters (who are diploid) while unfertilized eggs produce sons (who are haploid).
In a colony, the queen is equally related to her sons and daughters (r
Parent-offspring
conflict over sex
ratios in social
hymenoptera
0.5), so in
theory she should prefer a 1:1 sex ratio in her reproductive offspring. However, the
workers will prefer a different sex ratio in their mother's reproductive offspring. If
the queen is singly-mated, then the workers are three times as related to their
reproductive sisters (r
=
0.25). Therefore, the optimal
sex allocation from the workers' perspective is 3:1 in favour of reproductive sisters
(Trivers & Hare, 1976).
Who wins the conflict? Workers can bias sex allocation in their favour by selectively
destroying male eggs, and sometimes their 3:1 optimum is the outcome. In other cases,
however, sex ratios are closer to the queen's optimum and sometimes the outcome is a
compromise intermediate sex ratio, so either party can win depending on circumstances
(Ratnieks
et al
., 2006).
=
0.75) as to their brothers (r
=
Conflicts during pregnancy
If parental and offspring optima cannot be measured precisely, parent-offspring conflict
can, nevertheless, be shown by demonstrating that traits have antagonistic lifetime
fitness consequences for parents and offspring. Recall that we used the same argument
to demonstrate male-female mating conflicts (Chapter 7) where, for example, a male
trait, such as grasping genitalia, was counteracted by a female trait, such as spines to
resist male grasping. Similar antagonistic evolution has been demonstrated during
pregnancy in mice, where genes in offspring function to gain extra resources from the
mother, while genes in the mother function to resist offspring demand (Haig & Graham,
1991; Haig, 2000).
To understand this conflict, we first need to introduce the concept of 'genomic
imprinting'. Most genes are expressed in the same way, whether they are inherited from
the mother or the father. However, imprinted genes behave differently depending on the
parent they come from. David Haig and his colleagues suggested that genomic
imprinting evolves in the context of parent-offspring conflict. In many species (like
Genomic
imprinting may
evolve from
parent-offspring
conflict
